To improve fuel economy, hybrid electric vehicles (HEVs) may utilize regenerative braking, in which an electric machine applies regenerative braking torque to the powertrain of the vehicle during driver-induced friction braking of the vehicle. The electric machine converts the resulting kinetic energy into storable electrical energy which may subsequently be made available for vehicle propulsion. Regenerative braking is one of the enablers of hybrid vehicle technologies. It has been found that 15%˜30% fuel economy improvements over a non-regenerative braking-capable vehicle can be achieved using regenerative braking.
During driver-induced friction braking of a vehicle, the vehicle brake controller may transmit a brake torque command to the brake booster, which applies friction braking torque to the brakes at the vehicle wheels to decelerate or stop the vehicle. Simultaneously, the vehicle system controller may transmit a regenerative braking torque command to the hybrid powertrain to initiate regenerative braking. The regenerative braking torque command may dictate the magnitude of the regenerative braking torque which is applied to the hybrid powertrain to effect regenerative braking. The regenerative braking torque command may subsequently be reported to the vehicle brake controller to indicate the point at which ramp-out, or reduction and abatement, of the regenerative braking torque has begun. In turn, the vehicle brake controller may use both the driver torque command and the regenerative braking torque command to obtain the brake torque command which induces the brake booster to apply friction braking to the brakes.
At the onset of friction braking, there may normally be a slight delay in the accumulation of friction braking torque which the brake booster applies to the brakes at the vehicle wheels. This booster torque buildup delay may cause rough vehicle deceleration trends during the delay period, as illustrated in FIG. 1. It can be observed that at a low speed threshold (around 17.4 s) where regen is not desired, the powertrain torque starts ramping out, whereas the brake pressure starts ramping in, to compensate for reducing regen and satisfy the driver's brake request. However, due to the time response characteristics of the brake booster, the friction brake pressure starts ramping in, with a 120 ms delay. This delay, which is generally followed by an overshoot in booster pressure, causes relatively rougher deceleration trends, as can be verified from the deceleration plot. The delay is the time rate of change of deceleration and can reach 0.13 g/s, which may be manifested as a jerk that can be felt by professional drivers.
Typical booster torque buildup delay periods are on the order of 100-200 ms with 5 bar maximum overshoot.
The regenerative brake torque ramp down may be delayed as a solution, i.e. to compensate the brake booster delay, but reporting the delayed regenerative braking torque to the brake module may further increase friction brake ramp up delay as the friction brake ramp in is computed within the brake module by subtracting the regenerative braking torque from the total driver brake request.
Therefore, it may be desirable to report the undelayed, or raw, ramp-out of the regenerative braking torque to the vehicle brake controller at the onset of friction braking. Inducing such a delay in regenerative braking torque and instead of reporting this delayed regenerative braking torque, reporting the undelayed, or raw, regenerative braking torque to the brake module may yield synchronous ramp-up of friction braking to compensate for and reduce the effect of the booster torque buildup delay, eliminating or reducing rough deceleration trends which would otherwise occur during the booster torque buildup delay period.